Method of Making Green-Emitting Borate Phosphors

ABSTRACT

There is described a method for the preparation of a green-emitting terbium and cerium co-activated gadolinium magnesium pentaborate phosphor that utilizes a hydrated magnesium hexaborate as a boron source. The hydrated magnesium hexaborate preferably may be represented by the formula MgB 6 O 10 .XH 2 O where X is from 4 to 6, preferably 4.8 to 5.5, and more preferably about 5. The hydrated magnesium hexaborate is combined with oxides of Gd, Ce, and Tb, and at least one magnesium compound selected from MgCl 2 , MgF 2 , and MgO, and then fired in a slightly reducing atmosphere to form the phosphor. The method results in a greater homogeneity of the fired cake and subsequently a higher brightness. In addition, the method preferably requires only one firing step and provides very little or no sticking of the fired cake to the firing boats.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/892,326, filed Mar. 1, 2007, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The use of terbium and cerium co-activated green-emitting phosphors inmercury vapor discharge lamps for fluorescent lighting applications iswell established. The most commonly used of these phosphors includeLaPO₄:Ce,Tb, (Ce,Tb)MgAl₁₁O₁₉, and (Gd,Ce,Tb)MgB₅O₁₀ (referred to hereinas CBT). These green-emitting phosphors are typically mixed with ared-emitting phosphor such as Y₂O₃:Eu and a blue-emitting phosphor suchas BaMgAl₁₀O₁₇:Eu or Sr₅(PO₄)₃:Cl,Eu to form a blend which emits with anoverall white color when excited by the 254 nm radiation generated bythe mercury vapor discharge.

U.S. Pat. No 4,319,161 describes phosphors with the general composition(Y,La)_(1-x-y-z)Ce_(x)Gd_(y)Tb_(z)(Mg,Zn)_(1-p)Mn_(p)B₅O₁₀. The methodof producing these pentaborate phosphors involves dry mixing oxides ofthe rare earth elements, the oxide or hydrated carbonate-hydroxide ofmagnesium, manganese carbonate, the oxide of zinc and boric acid, andthen subjecting the mixture to two or three firings in a weakly reducingatmosphere. In U.S. Pat. No. 6,085,971, Tews et al. describes thatimproved brightness, processing, and stability under the influence ofshort-wave UV radiation may be achieved in luminescent metaboratephosphors of the formula(Y,La)_(1-x-y-z)Ce_(x)Gd_(y)Tb_(z)(Mg,Zn,Cd)_(1-p)Mn_(p)B_(5-q-s)(Al,Ga)_(q)(X)_(s)O₁₀, in which X is Si, Ge, P, Zr, V, Nb, Ta, W, or acombination thereof. Still, the synthesis method involved firing at twotemperatures, often with a comminution step in between.

Predictably, methods that require repeated grinding and firing steps arelabor intensive which generally means higher manufacturing costs.Furthermore, the use of large quantities of volatile boric acid leads tofurnace contamination and generates a significant waste stream.

Unlike the above methods, U.S. Pat. Nos. 4,719,033 and 5,068,055describe a single-step firing process for making europium-activatedstrontium tetraborate, SrB₄O₇:Eu, a UVA-emitting phosphor. The processinvolves adding a SrCO₃/Eu₂O₃ mixture to an H₃BO₃ slurry at >90° C. toform a (Sr,Eu)B₆O₁₀.5H₂O precipitate along with excess SrCO₃/Eu₂O₃ in a2:1 ratio. The precipitate is then fired to yield the SrB₄O₇:Euphosphor. No boric acid is used in the firing step. The hydratedprecipitate is fired after drying without adding additional compounds.

Thus, it would be an advantage to have a simpler process for making aCBT phosphor.

SUMMARY OF THE INVENTION

It is an object of this invention to obviate the disadvantages of theprior art.

It is a further object of this invention to provide an improved methodof producing a terbium and cerium co-activated gadolinium magnesiumpentaborate phosphor.

In accordance with one aspect of the invention, the terbium and ceriumco-activated gadolinium magnesium pentaborate phosphor has a compositionthat preferably may be represented by the general formula(Gd_(1-x-y)Ce_(x)Tb_(y))MgB₅O₁₀ where x has a value from 0.02 to 0.80and y has a value from 0.01 to 0.40, and x+y<1. More preferably, x has avalue from 0.1 to 0.4 and y has a value from 0.02 to 0.2.

In accordance with another aspect of the invention, the method of thisinvention involves the use of a hydrated magnesium hexaborate as a boronsource, preferably in place of boric acid. The hydrated magnesiumhexaborate preferably may be represented by the formula MgB₆O₁₀.XH₂Owhere X is from 4 to 6, preferably 4.8 to 5.5, and more preferably about5. The hydrated magnesium hexaborate is combined with oxides of Gd, Ceand Tb, and at least one magnesium compound selected from MgCl₂, MgF₂,and MgO, and then fired in a slightly reducing atmosphere. Preferably,the mixture is fired at a temperature from about 1020° C. to about 1060°C. in a 99% N₂/1% H₂ atmosphere to form the pentaborate phosphor. Apreferred firing time is from 3 to 4 hours.

In a preferred method, the hydrated magnesium hexaborate is prepared bydissolving boric acid in water to form a boric acid solution, heatingthe boric acid solution to a temperature of about 90° C., addingmagnesium carbonate to the boric acid solution, reducing the temperatureof the boric acid solution to within a lower temperature range of fromabout 35° C. to about 70° C., and maintaining the solution within thelower temperature range for at least about one hour. Preferably, theboric acid solution contains about 6.0 to about 12.0 millimoles of boricacid per 1.0 milliliter of water and about 1.0 to about 2.0 millimolesof magnesium carbonate per 1.0 milliliter water is added. Morepreferably, the boric acid solution contains about 9.0 to about 10.0millimoles of boric acid per 1.0 milliliter of water and about 1.5 toabout 1.67 millimoles of magnesium carbonate per 1.0 milliliter water isadded. The molar ratio of H₃BO₃ to MgCO₃ used to produce the precipitateis preferably 5.75 to 6.25 and more preferably about 6.0.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the emission spectra between 475 nm and 635 nm of astandard (Gd_(0.62),Ce_(0.23),Tb_(0.15))MgB₅O₁₀ phosphor and inventivephosphor sample 2-8.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawing.

The method of this invention uses a hydrated magnesium hexaborate as aboron source in the preparation of terbium and cerium co-activatedgadolinium magnesium pentaborate phosphors. The hydrated magnesiumhexaborate is preferably prepared as a precipitate which is dried toremove residual liquid water. In general, after drying, hydratedmagnesium hexaborate preferably may be represented by the formulaMgB₆O₁₀.XH₂O where X is from 4 to 6, preferably 4.8 to 5.5, and morepreferably about 5. The number of waters of hydration present in thedried material is dependent upon the drying conditions. In particular,above 200° C., the material starts to lose waters of hydration. Themajor phase after drying is typically MgB₆O₁₀.5H₂O. A typical secondphase MgB₆O₁₀.6H₂O tends to change to the pentahydrate form duringdrying.

The hydrated magnesium hexaborate is combined with oxides of Gd, Ce andTb, and at least one magnesium compound selected from MgCl₂, MgF₂, andMgO, and then fired in a slightly reducing atmosphere. This is differentfrom the prior art methods which combine separate oxide powders andmagnesium oxide with a large amount of boric acid which can contaminatethe furnace. The present method results in a greater homogeneity of thefired cake and subsequently a higher brightness. In addition, the methodmay be accomplished in one firing step and provides very little or nosticking of the fired cake to the firing boats.

The reported literature syntheses of MgB₆O₁₀XH₂O (X=5, 6, 7, and 7.5)were directed at the preparation of single crystals for structuraldetermination, although the structure has not been published. See,Lehmann and Rietz, Z. Anorg. Allg. Chem., 350, 168-176 (1967) andLehmann and Papenfuss, Z. Anorg. Allg. Chem., 301, 228-232 (1959). Atypical synthesis included small quantities and lengthy reaction times.For example, 1.5 grams of MgO and 60 grams of H₃BO₃ were agitated in 150ml of H₂O at 80° C. for 15 to 20 days to produce single crystals ofMgB₆O₁₀.7H₂O. Neither the small quantities nor the lengthy reactiontimes are desirable for commercial manufacturing. However, it wasdetermined that by using magnesium carbonate instead of magnesium oxideit was possible to manufacture commercial quantities of MgB₆O₁₀.XH₂O.

As used herein, the term “magnesium carbonate” and its general chemicalformula “MgCO₃” are to be broadly construed to include more complexhydrated carbonate forms such as Mg₅(CO₃)₄(OH)₂(H₂O)₄.

In one alternative embodiment, the hydrated magnesium hexaborate isformed by first dissolving about 6.0 to about 12.0 millimoles of boricacid per 1.0 milliliter of de-ionized water. The slurry is agitated andheated to about 90° C. Secondly, about 1.0 to about 2.0 millimoles ofmagnesium carbonate per 1.0 milliliter of de-ionized water are slowlyadded into the heated solution and the precipitate is digested for up to10 minutes at about 90° C. The temperature is then lowered to about 35°C. to about 70° C. and maintained within the lower temperature range forat least about one hour. The molar ratio of H₃BO₃ to MgCO₃ used toproduce the precipitate is preferably 5.75 to 6.25 and more preferablyabout 6.0. Observation indicates that hydrated magnesium hexaborate hasa significant solubility in water. Increasing the concentrations ofH₃BO₃ and MgCO₃ tends to increase the yield of the precipitate. However,when the concentrations are too high, the precipitate becomes overlythick and is difficult to further process. Reducing the finaltemperature to which the slurry is cooled also increases yield,presumably because the solubility of hydrated magnesium hexaboratedecreases as temperature decreases.

The present invention will be described in further detail with referenceto the following examples. However, it should be understood that thepresent invention is by no means restricted to such specific examples.

EXAMPLE 1 MgB₆o₁₀.5H₂O Preparation

A MgB₆O₁₀.5H₂O precipitate was prepared using 20 gallons of de-ionizedwater, 44.16 kilograms of boric acid, and 10.036 kilograms of MgCO₃. Theboric acid was added to the water, agitated, and heated to approximately92° C. before the MgCO₃ was slowly added. After the addition wascomplete, the samples were digested for 10 minutes at approximately 92°C. and then quickly cooled over 1 hour to 36° C. and further cooled to23° C. for 35 minutes. This material was dewatered in a filter crockovernight and then transferred to glass trays and dried at 250° F. forapproximately 24 hours and sifted through a 275 micron screen. Thewaters of hydration for the dried and sifted material were found to beX=5.16.

The most important differences between precipitation of MgB₆O₁₀.5H₂O andthe [2(Sr,Eu)B₆O₁₀.5H₂O+SrCO₃/Eu₂O₃] precipitate disclosed in U.S. Pat.Nos. 4,719,033 and 5,068,055 include the reactant concentrations and thedigestion temperatures.

MgB₆O₁₀.5H₂O is preferably prepared by reacting about 9 to about 10millimoles of H₃BO₃/ml H₂O and about 1.5 to about 1.67 millimoles ofMgCO₃/ml H₂O at about 90° C. and then digesting the precipitate for 3hours at decreasing temperatures with at least 1 hour of digestion at 50to 70° C.

The [2(Sr,Eu)B₆O₁₀.5H₂O+SrCO₃/Eu₂O₃] precipitate is prepared by reacting7.04 millimoles of H₃BO₃/ml H₂O and 1.59 millimoles of (SrCO₃/Eu₂O₃)/mlH₂O, of which ⅓ is unreacted excess, at about 95° C. and then digestingthe precipitate for 6 hours at >85° C.

Observation suggests that MgB₆O₁₀.5H₂O has a significant solubility inwater while (Sr,Eu)B₆O₁₀.5H₂O has only slight solubility in water. Theequipment used to make MgB₆O₁₀.5H₂O can be cleaned by merely soaking inwater. Increasing the concentrations of H₃BO₃ and MgCO₃ tended toincrease the yield of the precipitate, but when the concentrations aretoo high the precipitate becomes overly thick and is difficult tofurther process. Reducing the temperature during digestion alsoincreases yield, presumably because the solubility of MgB₆O₁₀.5H₂Odecreases as temperature decreases. Reducing the digestion temperatureappears to be more important than increasing the reactant concentrationsfor improving yield. Another method to increase yield requires theaddition of NH₄OH to raise the pH during digestion. This is thought tobe unnecessary when the concentrations of reactants are high and thedigestion temperature is slowly lowered. A further difference betweenthe two precipitation reactions includes the B/Mg and B/(Sr,Eu) ratios.For MgB₆O₁₀.5H₂O, there is little difference in yield when the B/Mgmolar ratio is 7.08:1 or 6.00:1 under similar reaction conditions. Thisis likely due to the fact that MgB₆O₁₀.5H₂O is partially water solubleand excess boron does not drive the precipitation to completion. For the(Sr,Eu)B₆O₁₀.5H₂O precipitation reaction, excess boron does appear todrive the precipitation to completion. Although the reactants aresimilar to those used for the [2(Sr,Eu)B₆O₁₀.5H₂O+SrCO₃/Eu₂O₃]precipitation process, higher concentrations and digestion at reducedtemperatures are important to increase the yield for this reaction. Thereduced digestion temperature has been experimentally determined to havelittle effect upon yield for [2(Sr,Eu)B₆O₁₀.5H₂O+SrCO₃/Eu₂O₃]precipitation and is, in fact, detrimental to the finished phosphorproperties.

EXAMPLE 2 (Gd,Ce,Tb)MgB₅O₁₀ Green-Emitting Phosphor Synthesis

In Example 2, several (Gd_(0.62),Ce_(0.23),Tb_(0.15))MgB₅O₁₀ phosphorswere synthesized with different ratios of magnesium halide compounds andthe MgB₆O₁₀.5H₂O precipitate. Except for sample 2-1, which contains bothMgB₆O₁₀.5H₂O and boric acid, MgB₆O₁₀.5H₂O was used as the boron sourcerather than boric acid. The comparative control sample, which wasdouble-fired at 1035° C. with a comminution step in between, used boricacid as the boron source and did not contain any magnesium halidecompounds. The detailed procedures for preparing the control sample aredescribed in U.S. Pat. No. 4,319,161, which is incorporated herein byreference. To optimize the blend ratio of raw materials, each sample wasformulated to contain the following molar ratios: 0.62 moles Gd, 0.23moles Ce and 0.15 moles Tb (from Gd₂O₃, CeO₂, and Tb₄O₇, respectively),1.025 moles of Mg (from MgO+MgCl₂+MgF₂+MgB₆O₁₀.5H₂O compounds) and 5.25moles B (from MgB₆O₁₀.5H₂O+H₃BO₃ compounds). Table 1 lists the rawmaterials, their molar ratios, the quantities used for inventive samples2-1 to 2-8, and the finished phosphor brightness.

The materials were weighed, added to a 500 ml plastic bottle, and thenblended on a roll mill and paint shaker. The mixture was then fired in asilica crucible for 3.0 hours at 1035° C. in a slightly reducing 99%N₂/1% H₂ atmosphere. The fired cake was wet milled with 5 mm YTZ beadsfor 90 minutes, washed, filtered, dried, and screened through 55 micronmesh to produce the (Gd_(0.62)Ce_(0.23)Tb_(0.15))MgB₅O₁₀ phosphor.Phosphor samples were packed into plaques and excited by 254 nmradiation from a mercury arc lamp discharge. The emission of each samplewas measured from 475-635 nm and compared to the standard(Gd_(0.62)Ce_(0.23)Tb_(0.15))MgB₅O₁₀ phosphor prepared according to U.S.Pat. No. 4,319,161. Emission spectra of sample 2-8 and the comparativecontrol sample are shown in FIG. 1. The data shows that the phosphorbrightness improves by increasing the molar concentration ofMgB₆O₁₀.5H₂O and by decreasing the molar concentration of boric acid.The data also shows that the phosphor brightness improves by replacingMgO with MgCl₂ and MgF₂ compounds.

TABLE 1 Relative Brightness of (Gd_(0.62)Ce_(0.23)Tb_(0.15))MgB₅O₁₀samples Rel. Mass of Raw Material, grams Brightness Sample Gd₂O₃ Tb₄O₇CeO₂ MgCl₂ MgF₂ MgO H₃BO₃ MgB₆O₁₀•5H₂O (475-635 nm), % Comparative 33.888.45 11.94 0 0 12.39 97.385 0 100 Control Mole Ratio 0.31 0.0375 0.23 00 1.025 5.25 0 2-1 33.88 8.45 11.94 0 0 3.33 13.91 76.33 101.8 MoleRatio 0.31 0.0375 0.23 0 0 0.275 0.75 0.75 2-2 33.88 8.45 11.94 0 0 1.810 89.05 102.2 Mole Ratio 0.31 0.0375 0.23 0 0 0.15 0 0.875 2-3 33.888.45 11.94 0 0.75 1.33 0 89.05 102.6 Mole Ratio 0.31 0.0375 0.23 0 0.040.11 0 0.875 2-4 33.88 8.45 11.94 1.15 0 1.33 0 89.05 103.1 Mole Ratio0.31 0.0375 0.23 0.04 0 0.11 0 0.875 2-5 33.88 8.45 11.94 0 1.50 0.85 089.05 103.3 Mole Ratio 0.31 0.0375 0.23 0 0.08 0.07 0 0.875 2-6 33.888.45 11.94 2.31 0 0.85 0 89.05 103.4 Mole Ratio 0.31 0.0375 0.23 0.08 00.07 0 0.875 2-7 33.88 8.45 11.94 4.32 0 0 0 89.05 105.0 Mole Ratio 0.310.0375 0.23 0.15 0 0 0 0.875 2-8 33.88 8.45 11.94 0 2.82 0 0 89.05 106.9Mole Ratio 0.31 0.0375 0.23 0 0.15 0 0 0.875

While there have been shown and described what are at present consideredto be preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications can bemade herein without departing from the scope of the invention as definedby the appended claims. In particular, although a single-step firingprocess is preferred, the process may less advantageously includeadditional firing steps.

1. A method of producing a green-emitting terbium and ceriumco-activated gadolinium magnesium pentaborate phosphor comprising: (a)combining a hydrated magnesium hexaborate with oxides of Gd, Ce, and Tb,and at least one magnesium compound selected from MgCl₂, MgF₂ and MgO toform a mixture; and (b) firing the mixture in a slightly reducingatmosphere to form the phosphor.
 2. The method of claim 1 wherein thehydrated magnesium hexaborate has a formula MgB₆O₁₀.XH₂O, where X isfrom 4 to
 6. 3. The method of claim 2 wherein X is 4.8 to 5.5.
 4. Themethod of claim 2 wherein X is about
 5. 5. The method of claim 1 whereinthe phosphor has a formula (Gd_(1-x-y)Ce_(x)Tb_(y))MgB₅O₁₀, where x hasa value from 0.02 to 0.80 and y has a value from 0.01 to 0.40, andx+y<1.
 6. The method of claim 5 wherein x has a value from 0.1 to 0.4and y has a value from 0.02 to 0.2.
 7. The method of claim 1 wherein themixture is fired at a temperature from about 1020° C. to about 1060° C.in a 99% N₂/1% H₂ atmosphere.
 8. The method of claim 1 wherein thehydrated magnesium hexaborate is formed by dissolving boric acid inwater to form a boric acid solution, heating the boric acid solution toa temperature of about 90° C., adding magnesium carbonate to the boricacid solution, reducing the temperature of the boric acid solution towithin a lower temperature range of from about 35° C. to about 70° C.,and maintaining the solution within the lower temperature range for atleast about one hour.
 9. The method of claim 8 wherein the boric acidsolution contains about 6.0 to about 12.0 millimoles of boric acid per1.0 milliliter of water and about 1.0 to about 2.0 millimoles ofmagnesium carbonate per 1.0 milliliter water is added.
 10. The method ofclaim 8 wherein the boric acid solution contains about 9.0 to about 10.0millimoles of boric acid per 1.0 milliliter of water and about 1.5 toabout 1.67 millimoles of magnesium carbonate per 1.0 milliliter water isadded.
 11. The method of claim 9 wherein the molar ratio of H₃BO₃ toMgCO₃ is 5.75 to 6.25.
 12. The method of claim 9 wherein the molar ratioof H₃BO₃ to MgCO₃ is about 6.0.
 13. The phosphor of claim 1 wherein theat least one magnesium compound is selected from MgCl₂ and MgF₂.
 14. Amethod of producing a green-emitting (Gd_(1-x-y)Ce_(x)Tb_(y))MgB₅O₁₀phosphor, wherein x has a value from 0.02 to 0.80 and y has a value from0.01 to 0.40, and x+y<1, the method comprising: (a) combining a hydratedmagnesium hexaborate with oxides of Gd, Ce, and Tb, and at least onemagnesium compound selected from MgCl₂, MgF₂ and MgO to form a mixture,the hydrated magnesium hexaborate having a formula MgB₆O₁₀.XH₂O where Xis from 4 to 6; and (b) firing the mixture at a temperature from about1020° C. to about 1060° C. in a slightly reducing atmosphere to form thephosphor.
 15. The method of claim 14 wherein x has a value from 0.1 to0.4 and y has a value from 0.02 to 0.2.
 16. The method of claim 14wherein the hydrated magnesium hexaborate is formed by dissolving boricacid in water to form a boric acid solution, heating the boric acidsolution to a temperature of about 90° C., adding magnesium carbonate tothe boric acid solution, reducing the temperature of the boric acidsolution to within a lower temperature range of from about 35° C. toabout 70° C., and maintaining the solution within the lower temperaturerange for at least about one hour.
 17. The method of claim 16 whereinthe boric acid solution contains about 6.0 to about 12.0 millimoles ofboric acid per 1.0 milliliter of water and about 1.0 to about 2.0millimoles of magnesium carbonate per 1.0 milliliter water is added. 18.The method of claim 16 wherein the boric acid solution contains about9.0 to about 10.0 millimoles of boric acid per 1.0 milliliter of waterand about 1.5 to about 1.67 millimoles of magnesium carbonate per 1.0milliliter water is added.
 19. The method of claim 17 wherein the molarratio of H₃BO₃ to MgCO₃ is 5.75 to 6.25.
 20. The method of claim 14wherein X is 4.8 to 5.5.
 21. The method of claim 14 wherein X is about5.
 22. The method of claim 19 wherein X is 4.8 to 5.5.
 23. The method ofclaim 19 wherein X is about 5.